1. Field of the Invention
This invention relates to an electronic display and more particularly to a variable speed clocking circuit for controlling the display.
2. Background of the Relevant Art
Electronic displays are well known in the art. As used herein, "electronic display" represents any electronically controlled terminal or monitor for displaying graphic and/or alphanumeric symbols. Electronic display includes any active or passive display device. Active devices include displays utilizing the various light-emitting processes such as, for example, cathodoluminescence, photoluminescence, electroluminescence, plasma decay and blackbody radiation. Cathodoluminescence includes cathode ray tubes (CRTs), photoluminescence includes colored gas discharge devices and fluorescent lamps, and electroluminescent devices include light-emitting diodes (LEDs). Plasma decay and blackbody radiation techniques are typically used in gas discharge panels and tungsten filament projection devices, respectively. Passive displays include displays controlled by light-absorption or light-reflection processes. Passive displays include electromechanical, electrochromeric, electropolarization and electrophoretic display techniques. The most popular passive display, utilizing electropolarization, includes liquid crystal displays (LCDs).
Electronic displays can present an object (alphanumeric character or graphic depiction) using various font geometries. For example, a CRT typically utilizes a large array of pixels arranged across the entire display area. One or more pixels may become illuminated to register a portion of the desired object. Thus, CRTs are often used to present alphanumeric or graphic objects. Passive displays, such as LCDs, often utilize seven or ten segment fonts. Instead of having an array of pixels arranged across the entire screen area, segments are arranged on a portion of the screen area. When one or more segments are illuminated, a corresponding alphanumeric symbol is formed. Various types of fonts become suitable depending upon the display technology chosen.
Regardless of the display technology chosen or the font being displayed, electronic displays are generally adjustable to suit human perception. The object can be moved horizontally or vertically by actuating one or more buttons arranged on the display housing. The viewing range, either horizontal or vertical range, may also be expanded or contracted to suit the viewer. Still further, many displays have switches which can vary the "visibility" of the object being displayed. "Visibility" is defined herein as quantifiable factors used to define ease by which the user perceives the object. There are two quantities which define visibility: display contrast and brightness.
Many conventional displays incorporate a memory which will store the desired optimal object position and viewing range after they have been set. Each time the display is thereafter turned on, the display position and field of view will be addressed from memory and fixed at the set position upon the screen. However, optimal visibility cannot easily be set and retrieved from memory. Visibility is a subjective standard which varies from user-to-user or from setting-to-setting. What might be perceived as an optimal contrast and brightness setting for one user is not optimal for another user. Furthermore, periodic changes to brightness and contrast may be needed depending upon whether the display is used in a bright or dark room. Sunlit rooms may require a larger contrast ratio in order to make the object more discernable from the display background. Due to necessity, visibility must thereby be left readily variable in order for each user to "tune" the setting to fit the particular surrounding or user's liking.
Light-emitting displays such as LEDs, plasma, CRTs, and vacuum fluorescent display measure luminance as the number of foot-lambert emitted. The amount of emission is termed luminance, and luminance varies across the display depending upon whether emission is from the object or from the background. The psychological interpretation of luminance is often termed "brightness". Brightness for a passive (non-light emitting display) is usually represented as a percentage of the brightness of a standard white material. Passive displays will reflect or absorb different amounts of light back to the eye (or photometer). For example, a display having no print will reflect a percentage of standard white. The printed portion will reflect a lesser percentage of standard white. The amount of reflection determines the amount of brightness (as perceived by the eye). The difference between the reflected background and the reflected printed portion can therefore be quantified in terms of a contrast ratio.
The contrast ratio is generally considered one of the most important visual characteristic of a display. The sole function of a display is to convey information by modifying an array of pixels or segments upon a screen. The contrast ratio indicates the amount of difference between a pixel or segment within an illuminated object area and a pixel or segment within the display background. The contrast ratio is thereby used to discriminate between, for example, a pixel that is fully on and a pixel that is fully off. Contrast ratio is often defined in simple terms as follows: ##EQU1## Luminance is often defined in terms of the amount of luminance per square area, or lux. Thus, contrast ratio is determined by measuring the ratio of the on luminance and off luminance per square area. Luminance of the on area is defined as that area through which light is emitted (active displays) or that area through which light is reflected or absorbed (passive displays).
As mentioned above, numerous visibility settings (contrast and brightness) may be required each time the display is used. In most cases visibility need only be changed slightly in order to take into account slight changes in human perception and relatively slight changes in surrounding lumination. However, if visibility must be drastically changed, it is important that it can be quickly changed with precision to the exact, optimal setting.